From a branding guy... yeah, using the same logo and font-face as TM is very confusing as to who owns what, and even borders on infringing. Yet you want to be unofficially connected to them - by safe association. At the same time rebranding, particularly when you've been using an established identity (in this case since 2007 or so), is hardly ever recommended.
For TMC it might make sense in making something similar, yet not too different (keep some of the existing DNA)... but more importantly, creating a tagline that creates the strong association that is needed. Something like, "Engaging forums for owners, enthusiasts, and fans of Tesla Motors".
imho